Extracellular vesicles are membrane enclosed vesicles released by all cells. Based on the biogenesis pathway different types of vesicles can be identified: (1) Exosomes are formed by inward budding of late endosomes forming multivesicular bodies (MVB) which then fuse with the limiting membrane of the cell concomitantly releasing the exosomes. (2) Shedding vesicles are formed by outward budding of the limiting cell membrane followed by fission. Finally, (3) when a cell is dying via apoptosis, the cell is desintegrating and divides its cellular content in different membrane enclosed vesicles termed apoptotic bodies. These mechanisms allow the cell to discard waste material and were more recently also associated with intercellular communication. Their primary constituents are lipids, proteins and nucleic acids. They are composed of a protein-lipid bilayer encapsulating an aqueous core comprising nucleic acids and soluble proteins. Currently, extracellular vesicles that are released in bodily fluids (e.g. blood, urine, saliva) receive a lot of attention as possible biomarkers for disease detection and progression, e.g. tumor growth and metastasis. Substantial efforts go into developing techniques suitable for extracellular vesicle identification. Identifying the origin of extracellular vesicles is typically done using biomolecular characterization techniques to determine the protein, nucleic acid and lipid content. One interesting alternative approach is the use of Raman spectroscopy as the Raman spectrum of extracellular vesicles may reveal their composition—and therefore their origin—in a label free manner.
To maximize the impact of current cancer treatments it is advantageous to detect carcinogenic cells in an early stage. To this end, the discovery of sufficiently sensitive and specific biomarkers is of foremost importance. Recently, circulating extracellular vesicles, especially exosomes, have emerged as a potential new class of biomarkers for early detection and treatment monitoring in cancer and other diseases.
Extracellular vesicles are of interest for diagnostic and prognostic applications as they contain molecules derived directly from the parent cell. In addition, they are fairly easily accessible as they are found in various body fluids (e.g. blood, salvia, urine, breast milk, ascites . . . ).
Currently, most extracellular vesicles based diagnostic approaches focus on one specific molecular component as a biomarker for the presence of diseased cells by elaborate genomic, proteomic, metabolomic and lipidomic studies. Examples are elevated levels of miR-21 in exosomes of hepatocellular cancer patients and the presence of EGFRvIII mutant proteins on extracellular vesicles derived from a specific glioblastoma subtype. Despite the fact that these techniques provide detailed molecular information, they require complicated and time-consuming protocols. Additionally, these analyses are performed on the overall extracellular vesicle population level which makes it less likely to find low abundant subpopulations. Considering that most cells secrete extracellular vesicles as part of their normal function, it is to be expected that the amount of vesicles derived from diseased cells is comparatively low. Accordingly, the detection of altered levels of low abundant components in a bulk analysis is quite challenging. Furthermore, it is becoming apparent that one cell type may release multiple subtypes of extracellular vesicles due to which bulk analysis is prone to missing specific subtypes or subtype ratios of vesicles.